1. Field of the Invention
The present invention relates to a multi-channel electrostatic sensor apparatus comprising a plurality of parallel channels each including a sensor circuit which is responsive to the change of an exterior capacitance for outputting a detection signal representing the change of the exterior capacitance.
2. Description of the Prior Art
A conventional electrostatic sensor uses a tank circuit having an exterior capacitance in its resonance circuit, thus providing an output signal whose frequency varies with the change of the exterior capacitance. This conventional electrostatic sensor is low in sensitivity. Recently, electrostatic sensors have become popular because of their relatively high sensitivity. (See U.S. Pat. No. 4,528,655). These electrostatic sensors use a resonance circuit whose resonant frequency is somewhat different from the oscillation frequency of an associated oscillator, and the capacitance of the resonance circuit is allowed to vary, thereby providing an amplitude-modulated signal. As shown in FIG. 1, it comprises oscillator circuit 1, resonance circuit 2, capacitance detector circuit 3, detection circuit 4 and amplifier circuit 5. The resonator of oscillator circuit 1 has a fixed oscillation frequency f1, whereas the resonance circuit 2 has a resonant frequency fo somewhat different from oscillation frequency f1, as seen from FIG. 2. The resonant frequency fo is allowed to change by .DELTA.f in response to the change .DELTA.C in a very small capacitance when detected by detector 3, thereby permitting conversion of the change .DELTA.C in the very small capacitance into corresponding voltage change .DELTA.V, which is amplified and derived.
There is a demand for detecting the change in each of a plurality of capacitances in an object to be tested, and processing a corresponding plurality of detection signals in different ways. For instance, a plurality of electrostatic sensors are used simultaneously to detect video signals from a plurality of disks in a video system, and a corresponding plurality of signals thus detected are subjected to parallel processing. In an attempt to carry out such parallel processing a parallel arrangement of series connections each comprising oscillator 1, resonance circuit 2, detection circuit 4 and amplifier 5 may be used. Assume that the oscillation frequency of oscillator is as high as 1 GHz, and that capacitance increment is detected at the sensitivity of 1.times.10.sup.-5 PF. Exterior disturbance and stray capacitances which are liable to appear in the electrostatic sensor circuit included in each channel, however, will make it difficult for each sensor circuit to have the same oscillation frequency. Oscillation frequency difference between adjacent channels causes mutual interference such as beat frequency between the oscillation frequencies of the channels. For instance, assume that oscillation frequency of a selected channel is "f", and that an oscillation frequency of adjacent channel is "F". Then, interference will be caused in the form of a beat frequency of F-f or F+f, adversely reducing the S/N ratio, preventing correct signal processing.
The oscillation frequency of oscillatin circuit 1 is selected to be 1 GHz, and the oscillation frequencies of adjacent channels can be selected to be apart from each other enough to to prevent the oscillation frequencies from mutual interference. It is, however, difficult to prevent the oscillation frequencies of each channel from producing beat frequencies when a plurality of channels are arranged. Very complicated production work is required to reduce the interference trouble using this remedy.
In order to eliminate such complicated production work the inventors proposed that a single oscillation circuit 1 be used by all channels in common. Specifically, a single oscillation circuit 1 distributes its oscillation frequency signal to the resonance circuits 2 of associated channels (See U.S. Pat. application Ser. No. 526,247). This proposed apparatus works well insofar as every channel is fixed and stationary. Assume that the channels which comprise a series connection of resonance circuit 2-to-amplifier 5 are integrally fixed to the video head of a video system and that video heads of a video system includes the channel comprising a series connection of resonant circuit 2 to amplifier circuit 5 and moves on the video disk of the video system and is conected to the resonance circuits 2 of all channels via cable or other conductors. If the cables are less flexible, movement of the video head will be adversely affected by lowering the video head's capability to detect video signals. Due to the angle of transmission of high frequency coaxial signals, cables are preferably used, but such coaxial cables are so heavy and less flexible that these cables, in fact, cannot be used. In an attempt to facilitate movement of the video head, shielded soft copper wires may be used. The wires, however, will change their shape while the video head moves on the disk, and accordingly the stray capacitance and impendance of the wires will change, thus reducing the amplitude of oscillation signals and causing distortion of the waveforms of oscillation signals. As a result, a very small change in capacitance cannot be detected. In an attempt to prevent a lowering of the oscillation signal, amplifier may be used to compensate for the reduction of the amplitude of oscillation signals. The use of such an extra amplifier, however, will cause undesired oscillation between oscillation circuit 1 and resonance circuit 2, reducing the stability of the capacitance detection system.